Optical Illumination System and Method

ABSTRACT

A system, method, and method of manufacturing directed to an optical device with an efficient optical illumination. The optical illumination can be provided by tilting a light source and using a refractive lens to direct the light onto a surface. Alternatively, the optical illumination can be provided using total internal reflection with a conical light pipe and a curvatured entrance and exit surface.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/033,427 entitled “An Optical Illumination System andMethod,” filed Dec. 27, 2001, which claims priority from provisionalU.S. Patent Application Ser. No. 60/290,268, for “An OpticalIllumination System and Method,” filed May 10, 2001, the disclosure ofwhich are incorporated herein by reference in their entirety.

BACKGROUND

A. Technical Field

The present invention relates generally to optical technology, and moreparticularly, to optical technology in an input device.

B. Background

Optical technology is used in many contexts, including in optical inputdevices. There are many different types of input devices, including amouse, a trackball, and a joystick. There are significant advantages tousing optical input devices over mechanical and opto-mechanical inputdevices. For example, mechanical or opto-mechanical input devices havemechanical components that are more susceptible to breakdown or wearout. Optical devices having only solid state components are lesssusceptible to such breakdown or wear out. However, one disadvantage ofsome optical input devices is increased power consumption, caused inpart by an inefficient illumination source or system. Illuminationrequires a precise angle of illumination and a sufficient optical powerto create a pattern on a surface (e.g., a table surface) that can thenbe captured by a photosensor. The pattern is the surface pattern itselfilluminated by the beam or the light and shadow of the surfacemicrostructure that is generated by the illumination beam impinging atthe appropriate angle. In conventional illumination systems, in order toachieve the desired illumination at the desired angle and the desiredoptical power, large power consumption is required due to an inefficientillumination system. This power consumption shortens battery life inwireless, optical pointing device systems.

As an example of an optical displacement system, consider an opticalmouse. The optical mouse includes a conventional illumination system.Conventional illumination systems consist of a light emitting diode(LED) and a double prism system. The double prism system consists of anentrance surface, a double prism, and three exiting facets approximatinga cylindrical concave exit surface. The entrance surface is apiano-convex lens shape linked to the double prism body that collectsthe LED light and collimates it. The double prism conducts the lightbeam to a target area on the table surface with the required incidenceangle. The cylindrical concave exit surface attempts to spread the lightevenly on the target area. An imaging lens creates an image of thelighted area on an optical sensor. The double prism system serves as alight conductor between the LED and the table surface (e.g. a table topor mousepad). Conventional illumination systems require that a totalinternal reflection (TIR) condition on be met. A TIR condition is metwhen an incidence angle of a light ray, for example, inside a plasticmedia interfaced with air, is larger than a critical angle resulting intotal internal reflection at the transparent material surface and norays are refracted outside the transparent material. However, rays thatdo not encounter the entrance surface or rays that do not satisfy the IRcondition within the double prism path are lost. In conventionalillumination systems, the LED is mounted on a printed circuit board(PCB) in a horizontal configuration on the component side of the PCB. Inthis conventional configuration, the LED can be easily soldered to thePCB simultaneously with the other electronic components. Thus, to directthe light to the target surface, the double prism is required to achieveboth the vertical distance and the required incidence angle.

Conventional illumination systems, using a double prism system, have along light path, multiple direction changes, and no way to recoverdiverging rays, thus, increasing loss and reducing efficiency.Furthermore, as the light source, which includes an LED die and LEDoptics, size is not a single point, it is not possible to accuratelyfocus all rays coming from the LED. There is a significant amount ofloss across this conventional system. Examples of four types of lossare: TIR loss, reflection/refraction loss, transmission loss, andcoupling efficiency loss. Coupling efficiency loss is caused by the factthat not all light from the LED can get into the double prism becausethe alignment of the LED with the entrance surface of the prism cannotbe perfect and the surface of the entrance lens of the prism is notlarge enough to collect all the viewing angle emitted by the LED. Eachof many intermediate parts contribute to this misalignment, for example,an LED package, an LED support, the PCB, and a mouse case. Due to theabove mentioned limitations, the intensity, the uniformity, and theposition of the illumination spot are degraded.

Therefore, there is a need for improving the illumination of an opticalinput device while improving the image signal power on a photosensor.Accordingly, it is also desirable to provide an optical input devicewith an efficient illumination source that helps reduce powerconsumption and increase battery life and illuminate the target areauniformly.

SUMMARY OF THE INVENTION

The present invention provides an efficient illumination system. Theillumination system can be used in optical input devices, for example,an optical mouse. The present invention includes an optical system thathas a conical light pipe with a curvatured (e.g. toroidal) entrance orexit surface (or “window”) in one embodiment and a refractiveillumination lens in another embodiment. For ease of discussion the term“or” as used herein means both inclusive or and exclusive or, i.e.,and/or.

In one embodiment, a refractive lens is used with a tilted light source.The light source can be a light emitting diode (LED) in the visible ornear infrared spectrums. The light source can emit light at any one ormultiple wavelengths. In alternative embodiments, refractive surfaces ofthe refractive lens can be replaced with a Fresnel surface or adiffractive optical element (DOE) surface. For ease of discussion, thepresent invention will be discussed with regard to a lens system thatmay comprise any one of the above optical surfaces or any combination ofthe above optical surfaces. It is understood that a refractive lensshall be used to refer to a lens that is either a refractive lens, aFresnel surface, a diffractive optical element (DOE), or any combinationof these lens types.

The light source can be angled relative to the printed circuit board. Inone embodiment, there is an opening in the printed circuit board for thelight source to protrude through. In another embodiment, the lightsource is mounted on a separate PCB. The lens system directs the lightemitted from the light source to a target area on a surface, e.g., atabletop or other surface. Typically, the PCB is parallel to the tablesurface. The table surface can be planar or curvatured, for example, inthe case of an optical trackball the surface is a curvatured surface. Inone embodiment, the light source is configured to be approximatelyparallel to the printed circuit board. In this embodiment, a conicallight pipe with a curvatured entrance surface or exit surface can beformed to direct the light emitted from the light source to the targetarea on the table surface. It is understood that a curvatured surfaceshall be used to refer to a surface with a toroidal shape, a sphericalshape, an aspherical shape, a cylindrical shape, or a spline shape. Theilluminated target area size is linked to the table surface seen by thesensor through any imaging lens plus safety margins for tolerances.

There are many benefits and advantages of the present invention. Oneadvantage is that less LED current is required for a higher opticalpower on the table surface due to an illumination yield gain. This helpsto prolong battery life for a wireless product. Another advantage isremoving a need for a high efficiency LED to compensate for aninefficient lighting system. This helps reduce costs because a lessefficient light source may be used. Another advantage is reducingmechanical dimensions for the system thereby increasing designflexibility and reducing cost. For example, there is a significantreduction in the size of the optical portion of an illumination system.The reduction in size permits a smaller lens part to be used, which usesless optical material in manufacturing, less injection time and asmaller mold, and therefore, reduces the cost. Another advantage is thatthe illumination area position robustness with respect to the targetarea is increased. Another advantage could be an increase in depth offield because a smaller aperture can be used with the imaging lens. Anincrease in depth of field allows for greater mechanical tolerances.Another advantage is a reduction in exposition time, the sensor beingilluminated with the required amount of energy in a shorter amount oftime. The time reduction factor is equivalent to the illumination yieldgain.

In one embodiment of the present invention, a refractive illuminationlens is used. It is noted that this embodiment of the present inventionprovides an overall lighting system that is refractive only, meaningthat TIR, which causes additional losses, is not used. In thisembodiment, the optical system length is reduced significantly by usinga tilted LED that is interfaced with a refractive lens instead of adouble prism or a light pipe. In this embodiment, the LED can be tiltedand moved closer to the target area. In one embodiment, the LED istilted such that it is not parallel to the PCB, for example placing theLED at a 20 degree to 30 degree angle to the PCB. The LED can bepositioned such that it protrudes down through the PCB. In coneembodiment, the refractive lens has a curvatured entrance surface and acurvatured exit surface.

In one embodiment of the present invention, losses in the system arereduced by the illumination light pipe, thus making it more efficient.The losses are reduced by the light pipe with a conical shape thatreduces the region or surfaces where rays are not under the TIRcondition. In one embodiment, instead of using a double prism, a conical(or cylindrical) light pipe is used. The conical light pipe has a largerentrance surface than exit surface. The large entrance surface combinedwith the light pipe function allows larger position errors for the LED.In one embodiment, a curvatured (e.g. toroidal) entrance surface or exitsurface is used. The toroidal shape means that the entrance surface orexit surface has at least two different radii) f curvature orthogonal toeach other, in a vertical and a horizontal plane. One embodiment has acurvatured surface at each end of the conical light pipe portion. Theconical section can be truncated by a first reflective surface. Thistruncation is advantageous because it allows the LED to be positionedhorizontally or obtains the required angle of incidence beam on thetarget surface. In another embodiment, a second reflective surface alsoacts to further direct the light toward the surface. The secondtruncation allows other positions of the LED and further increasesdesign flexibility. In one embodiment, the reflective surfaces combinedwith the light pipe direct most of the light out the exit surface,forming a twice-truncated cone. In one embodiment the reflectivesurfaces can be coated with a metallic covering to guarantee reflectionof rays not satisfying the TIR condition. In an alternate embodiment,the first reflective surface and the second reflective surface can beremoved when the LED is positioned at a predetermined angle.

In one embodiment, an illumination efficiency gain of at least two isrealized over a conventional illumination system by using, for example,a conical light pipe truncated by two reflective planes. This gain meanstwo times less current in the LED or half as much power needed for thesame illumination. For embodiments with the tilted LED, the efficiencyof the illumination system may increase to a factor of at least three.The length reduction of the complete lens system can be about 10millimeters (mm).

As described above, the benefits of the present invention include animproved battery life, for example for an optical cordless mouse, due toreduced power consumption and component efficiency gains. An efficientor powerful light source is not required with the present invention dueto increased efficiency in the illumination system. One embodiment ofthe present invention reduces the length of the optical system, whichenables greater industrial design flexibility. Using the presentinvention allows for the possibility of gaining depth of field byreducing the imaging lens aperture because there is more energy on thesurface. The present invention provides a much more robust system to themisalignment between the light source and the illumination lens byproviding enough energy on the surface. The present invention allows areduction of the exposure time of the sensor if the conventional (highefficiency) light source and the driving current are kept the same. Thepresent invention aims at illuminating the surface with a spot that ismore uniform.

As can be seen from the above description, the present invention may beapplied to many different domains, and is not limited to any oneapplication. Many techniques of the present invention may be applied toillumination in a number of optical displacement detection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are an illustration of a side view of one embodiment ofthe present invention that includes a refractive illumination lens witha plane, cylindrical, spherical, aspherical, or toroidal entrancesurface or exit surface.

FIG. 2 is an illustration of a second side view of one embodiment of thepresent invention that includes the refractive illumination lens, LED,target area, imaging lens, and sensor only.

FIGS. 3A, 3B, and 3C are an illustration of the construction method of aconical light pipe that includes zero, one, and two truncating planes.

FIG. 4 is an illustration of a side view of one embodiment of thepresent invention that includes a conical light pipe with a singletruncation plane.

FIG. 5 is an illustration of a side view of one embodiment of thepresent invention that includes a conical light pipe with two truncatingplanes and a curvatured entrance surface or exit surface.

FIG. 6 is an illustration of a top view of one embodiment of the presentinvention that includes a conical light pipe with two truncating planesand a curvatured entrance surface or exit surface.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the present invention is presented in thecontext of an optical illumination for optical displacement detectionsystem for use in, for example, a computer input device. In someembodiments, the principles disclosed may be implemented for use in anoptical mouse or an optical trackball. One skilled in the art willrecognize that the present invention may be implemented in many otherdomains and environments, both within the context of opticalillumination for optical displacement detection, and in other contexts.Different embodiments of the present invention are now described withreference to the figures where like reference numbers indicate identicalor functionally similar elements. Also in the figures, the left mostdigit of each reference number corresponds to the figure in which thereference number is first used.

Accordingly, the following description, while intended to beillustrative of a particular implementation, is not intended to limitthe scope of the present invention or its applicability to other domainsand environments. Rather, the scope of the present invention is limitedand defined solely by the claims.

Now referring to FIG. 1A, there is shown a side view of one embodimentof an optical lens system of the present invention that includes arefractive illumination lens. It is understood that the refractiveillumination system (e.g., with a flat, cylindrical, spherical,aspherical, or toroidal entrance or exit surface, or window) is alsoreferred to as a lens 135. FIG. 1A shows a light source 100, the lens135 having an entrance surface 110 and an exit surface 115, a printedcircuit board 105, a target area (or a concentration spot) 120 on asurface and an imaging lens 125. In one embodiment, the light source 100may protrude through an opening in the printed circuit board (“PCB”)105. Light emitted from the light source 100 enters the lens 135 throughthe entrance surface 110. The light exiting the exit surface 115 of thelens 135 forms a light beam 130 and is directed to a surface at thetarget area 120. The target area 120 is in line with imaging lens 125.The surface can be any surface, for example, a tabletop or surface, amouse pad, a paper, or any other surface. For each discussion theapplication will refer to a table surface as a generic representation ofall surfaces, including a ball surface for a trackball.

In an alternative embodiment, the light source 100 does not protrudethrough the PCB 105. In that embodiment, there is an opening in the PCB105 for the light emitted from the light source 100 to go through thePCB 105. In one embodiment, the lens 135 protrudes through the PCB 105.

The entrance surface 110 of the lens 135 is curvatured. In oneembodiment, the entrance surface 110 can be aspherical in shape tocollect as much light as possible. In another embodiment, the entrancesurface 110 of the illumination lens 135 can be matched with a shape ofthe LED tip so that a continuous media without changes of refractiveindex will result. The exit surface 115 bends the light such that it hasthe desired angle and focuses the light to produce an illumination spoton the target area that is as uniform as possible on the surface. In oneembodiment, the exit surface or the entrance surface can be ground todiffuse the light making it more uniform on the target area 120. The LEDdie has a contact point in the center causing a hole in the illuminationand a ground entrance 110 or exit surface 115 can avoid imaging the dieon the surface in some embodiments.

The entrance surface 110 is closest to the light source 100. Theentrance surface 110 can be symmetrical about the optical axis of theLED or it can be shifted by design. The entrance surface 110 can be usedto collect the light. The exit surface 115 is also a curvatured surfaceand can be configured to shape the light beam to compensate forelongation resulting from the oblique angle of the beam. Since the beamhits the target area at an angle, the corresponding dimension will beincreased, resulting in a light spot with a width and a height that areapproximately the same.

The entrance surface 110 of the lens 135 may be, for example, aspherical surface a cylindrical surface, a toroidal surface, or anaspherical surface and may be refractive, Fresnel, or DOE. Similarly,the exit surface 115 of the lens 135 also may be, for example,spherical, cylindrical, toroidal, or aspherical and may be refractive,Fresnel or DOE. The entrance surface 110 and the exit surface 115 eachrefract light. By adjusting the shape of both or either the entrancesurface 110 or the exit surface 115, the light beam emerging from thelens 135 can be shaped or tilted as needed. As an example, if onesurface is cylindrical, it will affect one dimension of the light beamfrom the light source 100. If the one surface is spherical, it willaffect both dimensions of the light beam from the light source 100 thesame way. If one surface is toroidal, it will affect two dimensions ofthe light beam from the light source 100, but in a different way. Theentrance and exit surfaces 110 and 115 can be parallel (in the sense oftwo plano-convex lenses linked together by their flat surfaces) orangled (a prism of wedge being added between the two flat surfaces). Inthe aligned configuration, the entrance and the exit beams axis will bethe same. In the angled configuration, the beam axis will be folded.

In one embodiment of the present invention, a refractive lens 135 isused. In one embodiment, the entrance surface 110 is an aspherical shapeand the exit surface 115 is a cylindrical shape. The aspherical entrancesurface gathers and focuses the light. The cylindrical exit surfacespreads the light evenly on the target area 120.

In one embodiment, the light source 100 of the present invention can bea light emitting diode (LED) emitting at approximately 630 nm. Inanother embodiment, the light source 100 can be any other light sourceat any wavelength in the visible spectrum or near the infrared spectrum.The light source can emit light at any one or multiple wavelengths. Thelens 135 can be made of many materials including any optical polymer orglass. Some examples of materials that can be used for the lens 135 arepolycarbonate, polystyrene, acrylic, polymethylmethacrylate, or anotheroptical plastic. In all embodiments, any material can be used such thatthe desired result of gathering and focusing light can be achieved.

A benefit of embodiments of the present invention using the lens 135 isthat they do not require the use of total internal reflection. By notusing total internal reflection to direct the light to the tablesurface, the system is more robust because there are fewer criticalsurfaces, which result in fewer errors or misalignments. Further, anoptical path for light can be significantly shorter than in lens systemsthat use total internal reflection, which also allows for potentiallyfewer chances of encountering flaws. The present invention preventscompounding light transmission errors that may exist when a preciseangle between the light source 100 and the entrance surface 110 of thelens 135 is not properly set.

Typically, a lens is close to the light source and symmetrical with anaxis of symmetry of the light source. In an implementation that relieson total internal reflection, the lens is sensitive to small variationsin alignment between the light source and the lens. However, someembodiments of the present invention do not rely on total internalreflection, including the embodiment shown in FIG. 1, and therefore, arenot as sensitive to misalignment between the light source and the lens.

Now referring to FIG. 1B, there is shown another embodiment of a sideview of the present invention. FIG. 1B shows another light source 140, aprinted circuit board 105, a wedge-shaped refractive lens 145, a targetarea 120, and an imaging lens 125. The embodiment shown uses a lightsource 140 with a narrow viewing angle. For example, the viewing anglecan be 15 degrees or less. Typically, the LED viewing angle isapproximately 30 degrees.

In this embodiment, since the light source has a narrow beam it is notnecessary to concentrate the beam and planar entrance or exit surfacescan be used. The wedge-shaped lens 145 functions to fold the light beamso that it reaches the target area at the desired angle. When the lightsource 140 with a narrow viewing angle is used, the entrance surface110′ may be flat. The lens 135 (shown in FIG. 1A) can be replaced withwedge-shaped lens 145. The wedge-shaped lens 145 bends a light beam axisso that it hits a target area 120 at a required angle. The target areais in line with an imaging lens 125. With a highly directive lightsource, an entrance surface 110′ of the wedge-shaped lens 145 issimplified, although a cylindrical or toroidal exit surface 115′ may beused to shape the light beam into a thin, but wide, shape. The angle 150pointing away from the PCB 105 can be any angle necessary to achieve thedesired deviation angle and avoids the TIR condition. For example, theangle 150 can be between 5 degrees and 35 degrees. For an LED with atypical viewing angle of 30 degrees, the wedge 135 can have an entranceor an exit surface that is curvatured, a combination of FIGS. 1A and 1B.Either the entrance surface 110′ or exit surface 115′ can be partiallyor totally ground.

Now referring to FIG. 2, there is shown a side view of one embodiment ofthe present invention, including the lens 135 with entrance surface 110and exit surface 115. FIG. 2 shows the light source 100, the lens 135,including the entrance surface 110 and the exit surface 115, the targetarea 120, the imaging lens 125, and a sensor die surface 205. In oneembodiment, the light source 100 shown is an LED. In one embodiment, theLED is at an angle relative to an upper surface of the PCB (not shown).Similarly to FIG. 1, light emitted from the LED, enters lens 135. In oneembodiment, the entrance surface 110 gathers and refracts the lightemitted from the LED 100. In one embodiment, the exit surface 115refracts the light gathered by entrance surface 110 and spreads thelight. The light is focused onto the table surface where the target area120 is located. The imaging lens forms an image of the table surfacewhere the target area 120 is located. The pattern on the table surfaceor the pattern of the microstructure of the surface is imaged by theimaging lens 125 on the sensor surface 205.

Now referring to FIG. 3A, there is shown a side view of a conical lightpipe. FIG. 3A shows a light source 100 and a conical light pipe 312. Theconical light pipe has an entrance surface 355 or exit surface 310. Theentrance surface 355 or exit surface 310 can be a curvatured surface.The conical light pipe is larger at the entrance than the exit. Theconical light pipe 312 shown is not truncated at all.

Now referring to FIG. 3B, there is shown a side view of the conicallight pipe shown in FIG. 3A with a single truncation plane. FIG. 3Bshows the light source 100, the entrance surface 355, an outline ofconical light pipe 312, first truncation plane 315, and a second conicallight pipe section 322. The first truncation plane is such that the TIRcondition is satisfied. In one embodiment, if the TIR condition is notmet for most of the contributing rays, the truncation plane can becovered with a metallic surface. For example, the incidence cone axis isbetween approximately 32 degrees and 90 degrees. The first truncationplane 315 is such that the rays inside the cone 312 are folded,respecting the TIR condition. When the rays encounter truncation plane315, they are reflected and continue in the conic section 322 until exitsurface 317. Conic section 322 is the mirrored image of conic section312 that is removed by the truncation.

Now referring to FIG. 3C, there is shown aside view of the singletruncated conical light pipe shown in FIG. 3B with a second truncationplane added. FIG. 3C shows the light source 100, the entrance surface355, an outline of the once truncated conical light pipe 322, the firsttruncation plane 315, a second truncation place 320, and a third conicallight pipe section 380. The resulting twice truncated conical light pipeis shown with hatch marks. The second truncation plane can be at anangle such that the TIR condition is satisfied, forming cone 380. In oneembodiment, the second truncation plane could also be covered with ametallic coating.

In one embodiment of the present invention, no truncation plane is used,as discussed above in reference to FIGS. 1 and 2. In another embodiment,one truncation plane is used, as discussed below in reference to FIG. 4.In a third embodiment, two truncation planes are used, as discussedbelow in reference to FIGS. 5 and 6. In the embodiments using zero, one,or two truncation planes a conical light pipe can be used. In theembodiments using zero, one or two truncation planes a cylindrical lightpipe can be used instead of the conical light pipe.

In one embodiment, the second truncation plane could be angled such thatconic section 360 points to the left instead of the to the right. Thelight pipe shown in FIG. 3C forms a “Z” shape. However, the light pipecould also be formed to for a “C” or a “U” shape. In these embodiments,the axis of the different conical sections are all in the same plane.Other embodiments are also possible.

Now referring to FIG. 4, there is shown a side view of one embodiment ofthe present invention including a once truncated conical light pipe. Itis understood that in any of the Figs. or description of a conical lightpipe, a cylindrical light pipe could be implemented in place of theconical light pipe. FIG. 4 shows a light source 100, an entrance surface455, a truncation plane 415, an outline of a conical light pipe 412, aonce truncated cone 422, and an exit surface 460. In this embodiment,the light source 100 is at an angle approximately vertical, such thatthe first truncation plane can reflect the light to a target area on atable surface.

Now referring to FIG. 5, there is shown a side view of one embodiment ofthe present invention including a twice-truncated conical light pipewith a curvatured entrance and exit surface. The conical light pipe canbe a circular cone or another shape cone, for example, a rectangularcone. The light pipe may comprise multiple sections. In one embodiment,the sections are not the same shape. For example, one section can be acircular cone shape and another section can be a rectangular cone shape.FIG. 5 shows a horizontal light source 100, a PCB 510, a curvaturedentrance surface 555, a sensor 125, reflective surfaces 515 and 520, acurvatured exit surface 560, a target area (or a concentration point)550 on a table surface 545, and an imaging lens 120. In the embodimentshown in FIG. 5, the light source 100 is used to emit light. In oneembodiment, the light source 100 is an LED. The light source 100 can beparallel to the PCB 510. The light emitted from the light source forms abeam 525. The light beam 535 can be directed towards the target area 550on a highly oblique angle using the reflective surfaces 515 and 520. Thelight diffused from the imaged area 550 is captured by the imaging lens120 to form an image of the target area on the sensor 125.

In one embodiment, the light source 100 of the present invention can bea light emitting diode (LED) emitting at approximately 630 nm. Inanother embodiment, the light source 100 can be any other light sourceat any wavelength in the visible spectrum or near the infrared spectrum.The lens shown in FIGS. 4-6 can be made of many materials including anyoptical polymer or glass. Some examples of materials that can be usedfor the lens are polycarbonate, polystyrene, acrylic,polymethylmethacrylate, or another optical plastic. In all embodiments,any material can be used such that the desired result of a light pipesatisfying the TIR condition is met.

In the embodiment shown, the light is gathered by the entrance surface555. The surfaces and truncation planes between the light source 100 andthe exit surface 560 forms a conical light pipe with the curvaturedentrance surface 555 and exit surface 560. The exit surface 560 can betoroidal, meaning the exit surface 560 may have two different radii ofcurvature in a vertical plane than in a horizontal plane. The truncationplanes 515 and 520 can form a truncated cone. The light beam diameter atthe entrance surface 555 is larger than the light beam diameter at theexit surface 560.

Now referring to FIG. 6, the conical light pipe with a curvaturedentrance surface 555 and exit surface 560 is shown from a top view.Similarly to FIG. 5, a light source 100 is used to illuminate a surface545 such as a table surface. The light is focused on the target area 550on the surface 545. Again, similar to FIG. 5, the reflecting surfaces515 and 520 form the twice-truncated conic light pipe. In oneembodiment, the exit surface 560 can be a toroidal exit surface.

From the above description, it will be apparent that the inventiondisclosed herein provides a novel and advantageous system and method forillumination in an optical device. The foregoing discussion disclosesand describes merely exemplary methods and embodiments of the presentinvention. As will be understood by those familiar with the art, theinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. For example, theinvention may be applied to other domains and environments, and may beemployed in connection with additional applications where opticaldisplacement or movement detection is desirable. Accordingly, thedisclosure of the present invention is intended to be illustrative, butnot limiting, of the scope of the invention, which is set forth in thefollowing claims.

1. An illumination system in an optical pointing device comprising: anentrance surface positioned to gather light from a light sourcepositioned at a first angle relative to a target surface; a light pipetruncated along a first truncation plane, the light pipe coupled to theentrance surface for directing the light gathered from the entrancesurface by reflecting at the first truncation plane meeting a totalinternal reflection condition for the light; and a curvatured exitsurface coupled to the light pipe, the curvatured exit surface directingthe light onto the target surface at a second angle relative to thetarget surface, the second angle different than the first angle.
 2. Thesystem of claim 1, wherein a portion of the light pipe isfrustro-conical shaped.
 3. The system of claim 2, wherein the light pipehas a larger entrance cross-section than an exit cross-section.
 4. Thesystem of claim 1, wherein the truncated plane is covered with a metalcoating.
 5. The system of claim 1, further comprising a second truncatedplane for further directing the light toward the exit surface at a thirdangle relative to the target surface, the third angle different than thesecond angle.
 6. The system of claim 5, wherein the second truncatedplane is covered with a metal coating.
 7. The system of claim 1, whereinthe light source is a light emitting diode.
 8. The system of claim 1,wherein the light pipe is made from an optical plastic.
 9. The system ofclaim 1, wherein the truncated light pipe is made from glass.